The field of this invention is that of removing blockages in remote subsea pipelines, typically from a subsea oil or gas well, between offshore platform installations, or from an offshore platform to the shore.
Hydrates are a porous solid which is formed primarily of water with a mixture of gases. It is effectively similar to crushed ice. There is a tendency for hydrates to form in the pipelines departing from a subsea gas well, especially on well startup.
The temperature of seawater at depths will often approach 32 degrees F., with the temperature in non-flowing pipelines being the same. When a subsea pipeline valve is opened, the gas expansion can cause substantial additional cooling. In these cold and high pressure conditions, hydrates of the gas and water can form quickly.
Frequently when the hydrate forms, it forms a blockage. The blockage will be somewhat porous. At that time, a high pressure will exist on the upstream side and a lower pressure will exist on the downstream side of the blockage. This means that additional gas will move through the hydrate and expand and therefore cool as it does. This means that not only can the expansion of the gas keep the formed hydrate cool, but can literally continue to grow additional hydrate blockage.
It is difficult to tell where the hydrates are actually located in deep water pipelines, especially when the pipelines are buried. They will most often be immediately downstream of a subsea valve or after a low point in a pipeline where water was able to collect.
Hydrates formed like this can last for weeks or months, with a substantial loss of gas flow and therefore revenue to the owner of the pipelines and subsea wells.
Paraffin can form blockages in pipelines by building up on the inner diameter of the cold pipelines as relatively warm oil circulates out of an oil well and cools as it flows down a subsea pipeline. As the layers of paraffin builds up on the subsea pipeline inner diameter, the inner diameter of paraffin becomes smaller and smaller. Ultimately a pigging device intended to clean the paraffin will cause the paraffin to separate from the inner wall of the pipeline and become a plug. In some cases, the paraffin will release from the subsea pipeline inner wall without a pig and cause a blockage. In either case, if the pressure in the pipeline is enough to move the plug along the pipeline, it will continue to collect additional paraffin as it moves until the length of the blockage cannot be moved by the available pressure.
Some attempts have been made to enter the end of the pipeline with a somewhat flexible string of coiled tubing to get to the blockage and wash it out. This is an expensive operation, and in some cases the blockages can be 10-20 miles away and are simply too far for coiled tubing to reach. Removal of the blockages can be further complicated if the pipe has bends in it which can make the operations difficult if not possible. In some of these cases the pipelines are cut and the end lifted to the surface to allow entrance into the pipeline. This is done at great expense and with significant threat of environmental pollution.
U.S. Pat. No. 6,939,082 shows a prior art method of remediation which shows a portable shoe handled by a remotely operated vehicle (ROV) which lands on the top half of the pipeline and circulates warm or hot water over the top half of the pipeline. This provides a benefit of mobility, but is limited in length. Sometimes the hydrates can be very long and impractical for a shoe in the range of six feet long to practically remediate. It is difficult for this type system to remediate a hydrate at the bottom of the pipeline.
In spite of the tremendous cost of loss of production and generally knowing approximately where the hydrates are likely to form, there has been no solution to address this in a practical and economic way.